Vehicle drive-trains of mobile working machines, in particular wheel loaders, are more and more often built with continuously variable power-split transmission devices which are made with a variator, a reversing transmission and range clutches. In the area of the reversing transmission spur gear stages or planetary gearsets are usually provided, such that the travel direction of the working machine is determined by the corresponding engagement of disk clutches. Driving ranges, within which a transmission ratio of the power-split transmission can be varied continuously by means of the variator, are obtained by coupled planetary gearsets or spur gear stages. To obtain driving range changes, as a rule disk clutches are used as the shifting elements. Furthermore, the variator is often designed as a hydrostatic transmission with hydrostatic units that can in each case be operated as a pump or a motor in combination with a power-splitting arrangement.
When the transmission ratio of the power-split transmission device is adjusted, for example starting from small transmission ratios toward larger transmission ratios, in each case one or more driving ranges are respectively passed through in part or totally, and for this the hydrostatic unit has to be swiveled several times over the complete driving range. For example, if two driving ranges which have to be completely passed through are considered, then around 0.5 to 1.0 seconds are needed for passing through the driving ranges and a further 150 to 200 ms are needed for the necessary driving range change, so that in total approximately 0.65 to 1.2 seconds pass by until a maximum transmission ratio is produced in the area of the transmission device. When a wheel loader drives onto a pile or heap, the load imposed from outside increases, in particular the deceleration, in less than 0.2 seconds. The control speed or speed of change of the transmission ratio in the area of the transmission device is therefore, disadvantageously, too low to avoid unacceptable decelerations of the rotational speed of a drive engine when driving onto a heap. Owing to the high control speeds of the hydrostatic unit required in combination with the unavoidable dead time associated with a driving range change, new methods are needed in order to satisfy the strict requirements for dynamics and for the protection of the engine and the transmission.
For example, DE 10 2013 222 693 A1 discloses a method for operating a vehicle drive-train with a drive engine and a power-split transmission device that can be brought, on the input side, into functional connection with the drive engine, which on its output side can be connected to a drive output, and whose transmission ratio in the area of a variator can be varied continuously, and with a shifting element which is arranged in the power flow of the vehicle drive-train between the drive engine and the drive output and whose transmission capacity can be varied continuously. The transmission capacity of the shifting element is varied as a function of the torque to be transmitted at the time in the drive-train between the drive engine and the drive output, and if the torque applied is greater than a defined threshold value, the shifting element changes to slipping operation.